56 PHILOSOPHICAL TRANSACTIONS. [ANNO 1781. 



1°. From April 14 d 8 h 48 m 52 s to April 19 d 7 h K> m s = 4 d 22 h 21 m 8 s = 12 

 revol. hence 1 revol. = Q h 5 1 m 45\6. 



2°. From April ig d 7 h 40 m to April 23 d 9 h 38 m = 4 d 2 h 28 m = 10 revol. 

 hence 1 revol. = g h 50 m 48 s . 



And taking both together, from April 14 d 8 h 48 m 52 s to April 23 d g h 38 m s 

 = Q A O h 4g m 8 s = 22 revol. hence 1 revol. = g h 51™ ig s .4. 



These several results are so exceedingly various, that it is evident Jupiter is not 

 a proper planet for the critical purpose of a comparative view of the diurnal mo- 

 tions ; nor can this great variety proceed from any inaccuracy in the observations; 

 for, Mr. H. thinks it is hardly possible to make a mistake in the situation of a 

 spot that shall amount to so much as 5 minutes of time. The observation of 

 April 23, 1 779, was made with a view to ascertain this point, when it was 

 found that 5 minutes of time made a sensible difference in the situation of a spot 

 when near the centre. 



By a comparison of the different periods it appears, that a spot which is carried 

 about in the atmosphere of Jupiter generally suffers an acceleration, or, which is 

 the same thing, performs its revolutions by degrees in less time than it did at 

 first ; which is agreeable enough to the theory of equatorial winds, since it may 

 probablv take up some time before a spot can acquire a sufficient velocity to go 

 as fast as those winds may blow. And, by the bye, if Jupiter's spots should be 

 observed in different parts of his year, and be found in some to be accelerated, 

 in others to be retarded, it would almost amount to a demonstration of his mon- 

 soons and their periodical changes ; but if his axis should not be inclined enough 

 to his orbit, to occasion such a change, they may probably always blow in the 

 same direction. 



Observations on Mars in the year \777- — Twenty-feet Newtonian reflector; 

 poiver 300. April 8, 7 h 30 m , observed 2 spots on Mars, with a bright belt or 

 partition between them. The belt not very well defined. At g h 30 m , the spots 

 were advanced, and more spotted parts visible. At 10 h the revolution of Mars 

 on his axis very evident. 



April 17, ten-feet Newtonian reflector ; power about 2 1 1 . At 7 h 50 m , two 

 bright spots, so luminous that they seemed to project beyond the disc. 



April 26, ten-feet reflector; power 211. At g h 5'", the spots on the planet 

 very taint. 



Observations on Mars in the year 1 779- — May gth, clock 15 s too fast; by 

 equal altitudes on the 14th of April, and by the transit of a star, it was found 

 to lose l m .45 per day. At 1 l h l m by the clock, a very remarkable dark spot not 

 far from the centre. At 1 l h 30 m the figures gone from the centre. 



May 1 1, clock 12' too fast. At 10 h I8 m , the same spot that was visible May 

 g, is on the disc, the darkest place being entirely south-east of the centre. At 



